In the development and exploitation of submarine hydrocarbon reservoirs, subsea wells are commonly used to extract the hydrocarbon fluids which can then be transported through submarine flowlines to above-surface hydrocarbon processing facilities at sea or on land. Depending on the characteristics of the hydrocarbon fluids, it may be necessary to maintain the temperature of the hydrocarbon fluids being transported in the flowline above a specific temperature thereby to prevent the flowline becoming blocked due to the solidification of wax, hydrates, or asphaltenes. The temperature of the hydrocarbon fluids in the flowline is normally maintained by surrounding the external surface of the flowline with a covering of material that has good thermal insulation properties. Depending on the level of thermal insulation required, this flowline covering can range from an external coating of polymer (e.g. polypropylene) to a pipe-in-pipe system, wherein the hydrocarbon-transporting pipe is placed coaxially within an outer carrier pipe and the annulus between the pipes is filled with thermally insulating material.
It is noted that conduits for hydrocarbons have previously been encased along parts of their length, either for simple bundling and/or for structural support. The term “pipe-inpipe” as used herein refers to a class of double-walled conduits with very high insulation performance, where inner and outer conduits are pre-formed into a rigid unit with a sealed annular space between the inner and outer conduits. The pipe-in-pipe conduit may be assembled from pre-formed pipe-in-pipe sections, or assembled from separate inner and outer pipe section directly into a longer pipe-in-pipe unit.
The pipe-in-pipe flowline may be arranged horizontally on the seabed, or may form part of a riser or riser tower of the type described in U.S. Pat. No. 6,082,391 [Stolt-Doris], or in copending international application WO 02/53869A [63752WO], not published at the present priority date. The pipe-in-pipe flowline may be formed as described in French Patent FR 2746891 (assigned to ITP), including provision of reduced gas pressure to improve insulation. The pipe-in-pipe flowline may be formed with an auxiliary conduit as described in application PCT/EP03/04178[64054WO], also not published at the present priority date. The contents of all these applications are incorporated herein by reference, especially for their teaching of pipe-in-pipe products and fabrication techniques.
The compositions of the hydrocarbon fluids found in some submarine reservoirs, particularly in the deep waters of the Gulf of Mexico, West Africa and Brazil, require thermal insulation values beyond those available from conventional pipe-in-pipe systems. During the operation of a subsea hydrocarbon field, the wells linking the reservoir to the above-surface hydrocarbon processing facility are often required to be closed, thus leaving non-flowing hydrocarbon fluids in the flowline. During these shut-in conditions the fluids are dependent on the thermal insulation system to maintain their temperature above that at which blockages may form. Under these shut-in conditions, even conventional pipe-in-pipe systems may not be able to provide sufficient thermal insulation to maintain the temperature of the hydrocarbons above that at which flowline blockages will occur, especially for a shut-in of extended duration.